Niobium base alloys have useful strength in temperature ranges at which nickel and cobalt base superalloys begin to show incipient melting. The melting temperature is in the range of 2300.degree. to 2400.degree. F. The use of higher melting niobium base metals in advanced jet engine turbine hot sections would allow higher metal temperatures than are currently allowed. The use of the niobium base alloy materials could permit higher flame temperatures and would permit production of greater power at greater efficiency. This is due in part to a reduction in cooling air requirements.
Commercially available niobium base alloys have high strength and high density but have limited oxidation resistance in the 1600.degree.-2400.degree. F. range. Silicide coatings offer some protection of niobium base alloys up to 2400.degree. F. but these coatings are brittle so that high stresses applied to the coated part could result in premature failure of the system.
Further, in devising alloy systems for aircraft engines, the density of the alloy is a significant factor. Commercially available niobium base alloys have high densities ranging from a low value of 8.6 grams per cubic centimeter for relatively pure niobium to values of about 10 grams per cubic centimeter for the strongest alloys.
Certain niobium-titanium base alloys have much lower densities of the range 6-7 grams per cubic centimeter. A group of such alloys are the subject matter of commonly owned U.S. Pat. Nos. 4,956,144; 4,990,308; 5,006,307; 5,019,334; 5,026,522; and Ser. No. 08/025,497, now U.S. Pat. No. 5,366,565. These alloys can be formed into parts which have significantly lower weight than the weight of the nickel and cobalt superalloys with densities in the range of 8.2-9.3 grams per cubic centimeter.
Thus, what is highly desirable in general for aircraft engine use is a structure which has a combination of lower density, higher strength at higher temperatures, good ductility at room temperature, and higher oxidation resistance. There is a need to devise metal-metal composite structures which have such a combination of properties.